Create a curved circle with precise dimensions
A curve with its control points precisely placed and a Screw modifier will do this.



Is this the shape you wanted?

Edit:

Sometimes the handles get swapped around, and that kinks the curve around on itself.  So move the handles around and see what effect that has on the curve to make sure they're coming out on the right sides.  If you move the control points (the handles get highlighted too) with G for grab, X, Y or Z for the axis and 0 or 1 or whatever you need, you can get them in a symmetrical place - you have control over that at least.  To get the handles arranged you can turn on Snap (the magnet icon at the bottom in Blender 2.79) and turn on Absolute Grid Alignment (the second icon along after Absolute Grid Alignment, you can move the handles to a fixed point on the grid so everything is properly aligned and symmetrical.  If you then want more or less curve you can move the handles to another grid point, keeping them symmetrical.